Although a Long Term Evolution (LTE) network is a fully packet switched network, voice services are indispensible and important services in a long time. To ensure the smooth development of high-quality voice services over the LTE network, various standardization organizations study actively and propose multiple voice service solutions.
One of the solutions to voice services over the LTE network is a circuit switched fallback (CSFB) approach in which a user resides in the LTE network, and a call falls back to a circuit switched (CS) domain when the user initiates or receives a voice service. An advantage of the CSFB is that operators just need to upgrade a mobile switching center (MSC) adjacent to the LTE network coverage instead of deploying an IP multimedia subsystem (IMS) network, and therefore, can provide voice services over the LTE network quickly. A disadvantage of the CSFB is that a call connection is established slowly, and user perceptions are poor. Another solution to the voice services over the LTE network is IMS-based voice, that is, VoLTE. Single radio voice call continuity (SR-VCC) is a VoLTE handover technology widely used by operators, and mainly resolves a problem of how to maintain voice service continuity when a single radio user equipment (UE) moves between the LTE network and a 2G/3G network, that is, a seamless handover when the single radio UE switches between an IMS-controlled Voice over Internet Protocol (VoIP) voice service (VoIP voice service) and a non-IMS controlled voice service (e.g. a CS domain voice service) or is handed over from the LTE network to the 2G/3G network. As compared with other handover technologies, the SR-VCC is more mature, and is used by most mainstream operators.
As shown in FIG. 1, the SR-VCC handover is triggered by a change of network quality/network coverage measured by a UE. In a VoLTE process, a UE keeps sending a measurement report to an evolved UMTS Terrestrial Radio Access Network (E-UTRAN), and the E-UTRAN determines, according to the measurement report, whether the UE needs to be handed over to the 2G/3G network.
However, as compared with the 2G/3G network, there are more coverage holes in the LTE network, and channel quality may suddenly change, for example, in an area such as an elevator, a subway, and an interior of a building. In the traditional SR-VCC handover, a handover decision is performed after a radio channel measurement report is obtained. At this time, the quality of a voice service may have been very poor, or a voice service has even been interrupted. Or, a normal SR-VCC process may not be completed in a short time. For a real-time data service such as the VoLTE, the relatively poor service quality or even interrupted service substantially affects user experiences.